We will continue the prospective, randomized, double-blind simultaneously -controlled study of the efficacy of aspirin in the prevention of postoperative thromboembolic disease following total hip replacement in patients over the age of 40 who have not had prior known thromboembolic disease or vein surgery in the lower extremity. All patients are being studied with three diagnostic techniques, namely; the fibrinogen uptake test, cuff impedance phlebography, and radiographic phlebography. In the second study, the efficacy of external pneumatic compression is being compared with that of warfarin as prophylaxis against thromboembolic disease following total hip replacement in patients over the age of 40 years who have had prior thromboembolic disease or vein surgery. The same diagnostic tools are being employed. All patients with pulmonary signs or symptoms are being studied with pulmonary ventilation and pulmonary perfusion scans, and when indicated, pulmonary angiography. The objectives are to evaluate the efficacy and safety of two promising methods of prophylaxis against postoperative thromboembolic disease, namely; aspirin compared to placebo in one group and external pneumatic compression compared to warfarin in the other group.